Which is best: a through-hull, transom mount, or shoot-through transducer? Traditional theory says the through hull wins, but how do we know for sure? And, just how big a difference does each type of transducer make? I was sick and tired of being told this and reading that—industry experts are long on talk and short on proof—so I figured it was time for some heavy-duty field work. Luckily, my boat had a 1,000-watt Simrad CA 44 fishfinder at the helm, and a through-hull transducer already installed. The folks at Airmar made me up a shoot-through and a transom mount with the correct plugs to try all three side by side, I filled the cooler with bait and ice (hey, if you’re going to be out there, you gotta bait up regardless of your mission!) and headed for the inlet. location, location, location To see how big a difference each ‘ducer made in different depths, I start out testing over an inshore wreck in 60’ of water. The through-hull is plugged into the Simrad and on the first pass I see the wreck in red sticking 20’ off the bottom with fish clearly visible, in reds and blues clustered just above and to the sides of it. I catch a few sea bass to prove the fish are for real, then pull the plug, switch over to the transom mount, and take another pass. Now the wreck has less definition, appearing to be a mere mass—a pile of something, made up of red and green blocky images with a few yellow blobs near by. When I switch wires again to try the shoot-through the object shrinks and flattens out into a fuzzy lump, the colors become even less distinguished, and what were separate targets indicating fish have become part of the whole. The difference between these pictures is as dramatic as the difference between first-generation digital cameras and today’s tack-sharp, 10 mega-pixel professional models.
In order to understand why the differences are so extreme, we need to understand what each unit is doing and why. Yeah, I know—technical gobbledegook is one big yawn. But don’t worry, we’ll keep it brief and simple.
The easiest way to visualize a transducer in operation is to think of it as both a speaker, and a microphone. The speaker shouts, then the mic listens. To shout the transducer produces an acoustic wave—a “ping”—which travels through the water at about 4,800’ per second. It has a specific wave form (long, penetrating waves with lower detail commonly at 50kHz low frequency, or short, low-penetration waves with higher detail commonly at 200kHz high frequency.) When the wave form bounces off anything with a different density than water, the transducer hears the difference, and shoots off a signal to the fishfinder’s processor. The processor then translates the anomaly into a picture.
How does the transducer hear? Inside its housing is a crystal, made of either lead zarconate or barium titanate. Specifics like transducer cone angle and frequency are dictated by the crystal’s shape. The crystal is soldered to wires, which lead to the transducer’s cable. Much like a telephone wire, the sounds the crystal “hears” are transmitted to the processor. Of course, the science geeks have a lot more to say about this stuff. But do you really need to learn about things like the Q factor, expressed as fr/^f@-3bd? I didn’t think so. Instead, let’s focus on finding those fish.
The view of the wreck proved traditional theory regarding through-hull superiority correct. Plus, once they’re installed they are maintenance-free. Unfortunately, they also require you to cut a hole in the bottom of your boat. They cause turbulence in water flowing aft which increases drag, they can’t be re-located easily, and they cost a lot to install. By the way, did we mention you have to cut a hole in the bottom of your boat?
Again holding to traditional theory, the transom mounted transducer gave me the second-best view of the wreck. The test model was Airmar’s P-66, one of the most popular options for outboard-powered boats. The P-66 is relatively inexpensive, easy to install, easy to move, and causes little drag or turbulence.
The third option, the shoot-through transducer, experiences a power drop when pushing through stuff other than water. Airmar came out with the P-79 to upgrade performance; it’s oil-filled, so there’s no energy-absorbing layer of adhesive between the face of the transducer and the bottom of the boat. The P7-9 has some pretty attractive advantages: it can be mounted anywhere in the boat, installs in a matter of minutes, can be adjusted to deadrise angle, and causes no drag or performance. There’s just one problem: remember that fuzzy lump on the bottom of the Atlantic?
Think of your transducer like a speaker and a microphone; one part shouts, the other listens. Wider (50-khz) beams penetrate deeper with less detail, while narrower (200-khz) beams penetrate less depth with greater detail.
Through-hulls: top-notch performance but you'll have a hole in your hull.
Transom mounts are the most common transducers, and they offer good performance.
Depth Matters
Okay: so we know how performance relates to transducer type in shallow water, looking at hard structure and fish. Plus, there are plenty of sea bass in the cooler at this point. Hey—we saw ‘em, we might as well catch ‘em, right? But what about in the deep? Would we experience the same results? To find out, we point the bow for Baltimore Canyon. Two hours later, I switch the unit over to 50kHz (remember, low wavelengths for deep penetration,) plug in the through-hull and take a few passes over the drop. In 723’, I see two red dots near the bottom. We send a few squid spiraling into the depths, and soon crank up a 10 pound tilefish. Yup, those marks were for real. Heading east the unit holds bottom down to 2,010’ until the screen finally goes blank.
The transom mount does pretty well, but still can’t match the through-hull’s performance. It holds bottom down to between 900’ and 1,000’ but is faint beyond 800’. The slower I go the better the performance, and the transom mount works best when I shift into neutral and drift.
Now—let’s see if the shoot-through works any better in low frequency mode than it did in high frequency. I switch plugs one more time, and…nope. It loses bottom between 660’ and 800’ never shows a separate target.
Watts Up???
Now, to test one more theory: Ron Weber, one of the big-wig Executive VP’s at Lowrance, told me that transducers had as great an effect on performance as power. I found this hard to believe. So to prove it he joined me on another test run—with the gear, computers, and codes necessary to manipulate a Lowrance LC-X 104 C’s power output from 50-watts to 1,000-watts on the spot. It was shouting into the depths through a massive grapefruit-sized 200/50 kHz through-hull transducer, on a temporary bracket mount. In 1,000’ the LC-X 104 C had no problem touching bottom with 1,000 watts. As he backed the power down, the bottom reading lost color definition and got a little spotty. But even at 50 watts, the reading held. Something seemed fishy; many expensive, powerful, 1,000-watt units can’t get a solid reading at these depths.
It turns out that, as proven during this afternoon of testing, pure power isn’t nearly as important as we give it credit for. Remember that big transducer? As Webber explained to me, when trying to penetrate the depths (and other factors such as frequency and receiver quality remain the same), doubling the transducer crystal’s diameter has the same effect as quadrupling the output power. This happens because the larger the transducer is, the more focused its beam is—think of a flashlight set to spotlight, instead of flood. So, a machine pushing 200-watts through an eight-inch transducer will see just as deep as a fishfinder pushing 800-watts through a two-inch transducer.
Of course, if you buy a fishfinder from a big-box store or a catalog, you’ll be stuck with the transducer the manufacturer supplies with the unit. For most 100-watt machines, as an example, that’ll be a one-inch crystal. You want your rig to be as capable as possible? Then make your purchase from a marine electronics specialist who can source the transducer separately, or look to Airmar. They have some transom mount models, like the new softball-sized TM258, which can give you a huge performance boost.
Back at the dock I don’t even need to look at my notepad; the test results are that obvious. Transducer choice clearly dictates how well your unit will perform. The through-hull provides the best performance, in all depths and situations. A transom mount comes in second, providing good shallow water definition and acceptable deep water performance. The shoot-through is last, and least, and while serious anglers may be interested in these for depth gauges they won't cut it for fishfinders. But size matters, too, and you can boost performance tremendously by up-sizing your transducer. Yes, all of these types have advantages and drawbacks. But if you want to catch more fish, it’s time to consider cutting a hole in the bottom of your boat.
Up-sizing your transom mount is a great way to boost performance; the big TM258 is five times as big as many transom mounts, and boosts performance incredibly.
Contact HookedOnFishingBoats.com by e-mailing lr@geareduppublications.com. Copyright 2009, by Geared Up, LLC.
